I recently bought the remaining stock of zerodur from an estate of a gentleman that ran a specialty optics shop. There's about 250lb of various sizes and shapes. Sizes range from 18 sq inch by <1" to 20" x 20" x 1.5".

Considering this stuff goes for $59/lb or more, $30 per pound seems appropriate + shipping. Cutting and other fabrication service is available but at extra charge.